In a display utilizing current-driven electro-optical elements, for example an organic light-emitting diode (OLED) and/or metal-oxide light-emitting diode (MOLED), in order to continuously run current through the OLED during a frame time, the time before a row of pixels is re-addressed in the subsequent frame, at least two thin-film transistors (TFTs) are provided at each pixel. In a typical 2T-1C configuration comprising two TFTs and one capacitor for one OLED pixel, the switch TFT turns on during programming time for the single row. The data voltage is applied at the data column line to set the storage capacitor to a particular voltage. When the switch TFT is turned “OFF,” the next program row is turned “ON” and programmed During the frame time, the storage capacitor maintains the data voltage. This data voltage sets the gate bias for the drive TFT that in turn sets the current through the OLED.
One of the biggest technological challenges for OLED technology is dealing with electrical non-uniformity and degradation mechanisms of the OLED and TFT. Over time, with electrical stress, the electrical properties of the TFT and OLED will degrade. If the OLED degrades wherein at the same voltage the OLED outputs a lower current, then the pixel brightness may be impacted. Non-uniformity and degradation of the TFTs may lead to poor display quality as a result.
To address the OLED degradation and non-uniformity, the drive TFT may be operated as a current source in the saturation regime. However, there is still the problem of drive TFT degradation and non-uniformity. The drive TFT has to be stable with minimal degradation and uniform wherein the TFTs in the display panel are matched. However, gate bias stress over time may shift the threshold voltage and mobility of the TFTs. To handle this, alternate pixel circuits have been proposed that utilize multiple TFTs to do circuit compensation of threshold voltage shifts. Such circuits may utilize three to more than six TFT pixel circuits that can compensate for the threshold voltage shift and/or variation. However, these circuits do not account for the mobility shift and/or variation.
In another approach, a current signal may be utilized at the data column lines to set the state at the individual pixels instead of using a voltage signal to control the state of the individual pixels. For a conventional 1T-1C liquid-crystal display (LCD) and 2T-1C OLED pixel circuits, the data column lines are operated with voltage data signals.
Some approaches to solving the threshold and mobility degradation and/or variation of the drive TFT involve applying current data signal to the column lines. This approach is often referred to as current programming. In these approaches a fixed current level may be applied through the drive TFT of the selected pixels in the program row. The storage capacitor will charge up to the specified gate bias of the drive TFT in order to achieve a predetermined current level. However, one of the challenges for pixel circuits with current-programming driving schemes is the charging time for low data currents involved with a low pixel brightness setting. The data current has to charge all the parasitic interconnect capacitances as it charges the storage capacitor. Low data current will take longer to charge, which may be difficult to do within a short row program time. As display sizes increase, the row program time decreases, but interconnect capacitance is larger. Thus, there may not be enough time to provide a full charge.
Most of the solutions to the low data current charging issue involve using some form of current scaling, where the data program current is higher than the actual current at the drive TFT. One such method is to use dimensional scaling of TFTs. Suppose a display has TFT “A” and TFT “B,” where TFT “B” is the drive TFT connected to the OLED and it has a lower width to length (W/L) ratio than TFT “A”. During the row program time, the data programming current runs through TFT “A” and charges the storage capacitor that is tied to gates of both TFT “A” and TFT “B.” Therefore the drive TFT “B” that is connected to the OLED will operate a current that is scaled by its lower W/L ratio. This pixel circuit will only work as long as TFT “A” and “B” are matched, which can be assumed since they are in close proximity to each other. However, they may not degrade at equal rates since TFT “B” is operated for a longer time than TFT “A” and is subjected to lower currents. Therefore, such a pixel circuit will only work for backplanes that have non-uniform but stable VT and mobility. Another issue with dimensional scaling is that scaling current by 10× for example may be difficult given the constraints of the pixel area allowed for higher resolution displays.
It will be appreciated that for simplicity and/or clarity of illustration, elements illustrated in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, if considered appropriate, reference numerals have been repeated among the figures to indicate corresponding and/or analogous elements.